CBSE Class 10 Light — Reflection and Refraction Complete Chapter Guide

In short: Light — Reflection and Refraction carries 7-8 marks in the CBSE Class 10 board exam. Master the mirror formula (1/v + 1/u = 1/f), lens formula (1/v − 1/u = 1/f), sign convention, ray diagrams for all six positions of concave mirrors and convex lenses, and Snell's law. Three well-practiced numericals can guarantee full marks in this chapter.

Light — Reflection and Refraction is one of the most important chapters in CBSE Class 10 Physics. It carries 7-8 marks and appears every year in the board exam. The chapter is split into two halves — reflection by mirrors and refraction by lenses — and both halves are numerical-heavy. Students who are comfortable with the formulas and sign convention consistently score full marks here.

This guide covers every concept, formula, ray diagram, and numerical type you need. For the full marks breakdown across all Science chapters, see the CBSE Class 10 Science chapter-wise marks weightage.

Laws of Reflection

When a ray of light strikes a smooth surface, it bounces back. This phenomenon is called reflection. Two laws govern it:

1. The angle of incidence is equal to the angle of reflection (angle i = angle r).
2. The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

These laws apply to all types of reflecting surfaces — plane mirrors, curved mirrors, and even irregular surfaces (though irregular surfaces cause diffuse reflection).

Spherical Mirrors — Concave and Convex

A spherical mirror is a part of a hollow sphere. If the reflecting surface is on the inner (concave) side, it is a concave mirror. If it is on the outer (convex) side, it is a convex mirror.

Key terms you must know:

Concave mirror — converging mirror. Used in torches, headlights, shaving mirrors, solar furnaces, and by dentists.

Convex mirror — diverging mirror. Used as rear-view mirrors in vehicles because they give a wider field of view and always form erect, diminished images.

New Cartesian Sign Convention

This is the foundation of every numerical in this chapter. Get it wrong and every calculation goes wrong.

• All distances are measured from the pole (P) of the mirror or the optical centre (O) of the lens.
• Distances measured in the direction of incident light (left to right) are positive.
• Distances measured against the direction of incident light (right to left) are negative.
• Heights measured upward from the principal axis are positive.
• Heights measured downward from the principal axis are negative.

Practical implications for mirrors:

• Object distance (u) is always negative (object is placed on the left).
• For concave mirror: focal length (f) is negative, and image distance (v) can be negative or positive depending on image position.
• For convex mirror: focal length (f) is positive, and image distance (v) is always positive (image forms behind the mirror).

Ray Diagrams for Concave Mirror — Six Positions of Object

Drawing correct ray diagrams is worth 2-3 marks. Use these three standard rays:

1. A ray parallel to the principal axis reflects through F.
2. A ray passing through F reflects parallel to the principal axis.
3. A ray passing through C reflects back along the same path.

Image formation by concave mirror:

Board Exam Tip: The question "draw a ray diagram when the object is placed between F and C of a concave mirror" is one of the most repeated questions. Practice drawing it neatly with all labels.

Mirror Formula and Magnification

Mirror formula:

1/v + 1/u = 1/f

Where v = image distance, u = object distance, f = focal length. All measured from the pole using the sign convention.

Magnification:

m = −v/u = h'/h

Where h' = height of image, h = height of object. If m is negative, the image is real and inverted. If m is positive, the image is virtual and erect. If |m| > 1, the image is enlarged. If |m| < 1, the image is diminished.

Refraction of Light

When light passes from one transparent medium to another, it changes direction at the boundary. This bending of light is called refraction. It happens because the speed of light is different in different media.

• When light goes from a rarer medium (air) to a denser medium (glass), it bends toward the normal.
• When light goes from a denser medium to a rarer medium, it bends away from the normal.

The refractive index of a medium is the ratio of the speed of light in vacuum to the speed of light in that medium: n = c/v.

Snell's Law

Snell's law gives the relationship between the angles and refractive indices of two media:

n1 × sin i = n2 × sin r

Or equivalently, the refractive index of medium 2 with respect to medium 1 is:

n₂₁ = sin i / sin r

Refractive indices of common materials:

A higher refractive index means the medium is optically denser and light travels slower in it.

Spherical Lenses — Convex and Concave

A convex lens is thicker at the centre and thinner at the edges. It converges light rays and is called a converging lens.

A concave lens is thinner at the centre and thicker at the edges. It diverges light rays and is called a diverging lens.

Key terms for lenses:

• Optical centre (O): Centre of the lens. A ray through O passes undeviated.
• Principal focus (F): Point where parallel rays converge (convex) or appear to diverge from (concave). A lens has two foci — F1 and F2 — on either side.
• Focal length (f): Distance from O to F.

Ray Diagrams for Convex Lens — Six Positions of Object

Use these three standard rays for convex lens:

1. A ray parallel to the principal axis passes through F2 after refraction.
2. A ray passing through F1 emerges parallel to the principal axis after refraction.
3. A ray through the optical centre O passes without deviation.

Image formation by convex lens:

Image by concave lens: For all object positions, a concave lens always forms a virtual, erect, and diminished image on the same side as the object (between F and O of the lens).

Lens Formula and Power of a Lens

Lens formula:

1/v − 1/u = 1/f

Sign convention for lenses:

• For convex lens: f is positive.
• For concave lens: f is negative.
• Object distance u is always negative.

Magnification for lenses:

m = v/u = h'/h

Power of a lens:

P = 1/f

Where f is in metres and P is in dioptres (D). A convex lens has positive power and a concave lens has negative power.

When multiple lenses are combined, the net power is:

P = P1 + P2 + P3 + ...

This is an important concept for 1-mark questions.

Solved Numericals

Numerical 1: Concave mirror

An object is placed 30 cm in front of a concave mirror of focal length 15 cm. Find the position, nature, and magnification of the image.

Given: u = −30 cm, f = −15 cm (concave mirror, both negative by convention)

Using mirror formula: 1/v + 1/u = 1/f 1/v + 1/(−30) = 1/(−15) 1/v = −1/15 + 1/30 = (−2 + 1)/30 = −1/30 v = −30 cm

The image is formed at 30 cm in front of the mirror (at C). The image is real and inverted.

Magnification: m = −v/u = −(−30)/(−30) = −1

The image is the same size as the object and inverted.

Numerical 2: Convex lens

An object 5 cm high is placed at a distance of 20 cm from a convex lens of focal length 15 cm. Find the image position, nature, and size.

Given: u = −20 cm, f = +15 cm, h = 5 cm

Using lens formula: 1/v − 1/u = 1/f 1/v − 1/(−20) = 1/15 1/v + 1/20 = 1/15 1/v = 1/15 − 1/20 = (4 − 3)/60 = 1/60 v = +60 cm

The image is formed 60 cm on the other side of the lens. Positive v means the image is real.

Magnification: m = v/u = 60/(−20) = −3

Height of image: h' = m × h = −3 × 5 = −15 cm

The image is real, inverted, and 15 cm tall (three times enlarged).

Numerical 3: Power of lens combination

A convex lens of focal length 25 cm and a concave lens of focal length 40 cm are placed in contact. Find the power and nature of the equivalent lens.

Power of convex lens: P1 = 1/0.25 = +4 D Power of concave lens: P2 = 1/(−0.40) = −2.5 D

Net power: P = P1 + P2 = 4 + (−2.5) = +1.5 D

The equivalent focal length: f = 1/P = 1/1.5 = 0.667 m = 66.7 cm

Since the net power is positive, the combination acts as a convex (converging) lens.

Common Mistakes in Board Exams

1. Forgetting sign convention. Always assign signs before substituting values. The most common error is using a positive value for u.
2. Mixing up mirror and lens formulas. Mirror formula has a plus sign (1/v + 1/u), lens formula has a minus sign (1/v − 1/u).
3. Wrong magnification formula. For mirrors it is m = −v/u, for lenses it is m = v/u. Note the negative sign difference.
4. Incomplete ray diagrams. Always draw at least two rays, label F, C or 2F, and clearly mark the image with an arrow.
5. Not stating the nature of image. After every calculation, state whether the image is real/virtual, erect/inverted, and enlarged/diminished.

Board Exam Important Questions

• Draw ray diagrams for concave mirror (object between F and C) and convex lens (object beyond 2F1). These two are the most repeated.
• Numericals on mirror formula and lens formula (one each is almost guaranteed).
• One-mark questions on power of a lens and its SI unit.
• "Why are convex mirrors preferred as rear-view mirrors?" — a very common 2-mark question.
• Differentiate between real and virtual images (table format preferred).

For the complete formula list across all chapters, refer to the CBSE Class 10 Science formulas complete list.

Frequently Asked Questions

How many marks does Light — Reflection and Refraction carry in CBSE Class 10?

This chapter typically carries 7 to 8 marks in the board exam. You can expect one 3-mark numerical (mirror or lens formula), one 2-3 mark ray diagram question, and one or two 1-mark MCQs. It is one of the most scoring chapters because the question patterns are highly predictable.

What is the difference between mirror formula and lens formula?

The mirror formula is 1/v + 1/u = 1/f (note the plus sign), while the lens formula is 1/v − 1/u = 1/f (note the minus sign). Also, the magnification formula differs: for mirrors, m = −v/u, and for lenses, m = v/u. Always check which formula applies before substituting values.

How do I remember the sign convention for mirrors and lenses?

Think of the mirror or lens as the origin of a number line. Everything to the left of the mirror/lens (where the object is) is negative. For mirrors, everything to the left of the pole is negative (including f for concave mirrors). For lenses, the object side is negative and the opposite side is positive. Heights above the principal axis are positive, below are negative.

Which ray diagrams are most important for the board exam?

The two most repeated ray diagrams are: (1) concave mirror with object between F and C, and (2) convex lens with object between F1 and 2F1. Also practice the diagram for a concave mirror with object beyond C and a convex lens with object at infinity. For each, draw at least two of the three standard rays clearly.

Can I use the same formula for convex and concave mirrors?

Yes, the mirror formula 1/v + 1/u = 1/f works for both concave and convex mirrors. The sign convention handles the difference. For a concave mirror, f is negative. For a convex mirror, f is positive. Apply signs correctly and the formula gives the right answer for any mirror type.

What is the power of a lens and why is it important?

Power of a lens (P = 1/f, where f is in metres) measures how strongly a lens converges or diverges light. It is measured in dioptres (D). Convex lenses have positive power, concave lenses have negative power. This concept is important because opticians prescribe spectacle lenses in dioptres, and questions on combining lenses using P = P1 + P2 appear frequently.

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